In modern communications networks, communications traffic is often carried on optical fibers. A plurality of transmitters may be connected to a first end of a fiber through an optical multiplexer to transmit a corresponding plurality of optical payload signals thereon. Each transmitter includes a semiconductor laser whose output optical intensity is modulated between a first predefined intensity representing a logical “0” and a second predefined intensity representing a logical “1” to generate an optical payload signal. Using a method called wavelength division multiplexing (WDM), the lasers transmitting on the fiber are chosen such that each laser generates a signal having a unique optical wavelength, permitting a plurality of payload signals to be optically multiplexed onto a single optical fiber without becoming mixed. An optical demultiplexer is connected to an opposite end of the fiber to demultiplex the plurality of optical payload signals onto a corresponding plurality of optical links for receipt by a corresponding plurality of receivers.
To manage such a communications network efficiently, it is necessary to monitor power of the optical signal, to ensure that the signal will be properly propagated through the optical fiber to be detected at a receiver. One method of facilitating optical power measurement is disclosed in U.S. Pat. No. 5,513,029 to Roberts, wherein an optical payload data signal is submodulated with a dither signal encoded with a pseudo random code, and having a submodulation depth which is maintained constant relative to mean optical power of the submodulated optical payload data signal. A pseudo random code is used to broaden the emission spectrum of lasers used in optical transmitters to reduce non-linear optical effects such as Stimulated Brillouin Scattering (SBS).
In addition, to effectively manage an optical system it is desirable to transmit communications and control information between nodes on an optical network. This is typically done by including communications and control information, also referred to as overhead data, in the payload data signal or by submodulating the payload signal with overhead data.
Thus, submodulation of an optical signal has been used for either power monitoring or for transmission of communications and control data. Typically, where submodulation is used for power monitoring, communications and control data is transmitted in payload data and where submodulation is used for payload data, power monitoring is not provided.
Generally, it is not possible to simply add the communications and control data to the pseudo random codes used for power monitoring as this can result in a submodulation depth greater than an allowable limit. Exceeding such a limit may limit the distance the light can travel in the fiber, requiring more optical amplifiers at shorter spacings.
Furthermore, while channel power information is directly measurable when the channels are separate, prior to WDM multiplexing or after demultiplexing, it is difficult to directly measure the power of individual channels of a WDM signal. One existing method of determining channel power of a WDM signal involves optically demultiplexing the WDM signal to retrieve individual optical payload signals, converting the individual optical payload signals into individual electrical signals and then measuring the power of each such electrical signal. However, this method requires the use of a relatively expensive optical demultiplexer and may not, therefore, be economical.
Thus, there is a need for a way to transmit power information and non-payload data in an optical signal without excessive depth of modulation and without interfering with the payload data, while facilitating economical power measurement.